Spark plug insulator



Patented June 23, 1936 UNITED STATES PATENT OFFICE 2,045,318 SPARK PLUG INSULATOR Richard Stribeck, Stuttgart, Germany, assignor to Robert Bosch Aktiengesellschaft, Stuttgart,

Germany 3 Claims. (Cl. 10612) Sparking plug insulators must have not only great mechanical strength but also a great resistance to changes of temperature. More especially, the stresses which occur in the interior 5 of the sparking plug insulator due to the varying temperatures during the use of the sparking plug must be avoided as far as possible. Known highly refractory materials, consisting of a single metallic oxide having a high fusing point, e. g. corundum, are therefore used for their manufacture. Such insulators are, however, very difiicult to manufacture, because they require very high burning temperatures which approximate to the fusing temperature of the corundum, and for which the customary tunnel kilns used for mass production are not suitable, and because it is almost impossible to avoid holes and other faults in the structure of the material. Highly refractory ceramic materials are also known, consisting of a metallic oxide as the base material and a clayey binder, so as to reduce the burning temperature and give a denser structure. It has hitherto been attempted to use such binders, the thermal expansion of which, in the non-plastic state, corresponds as far as possible to the thermal expansion of the base material, for example, corundum. Since, however the binder must also comply with other, and contradictory requirements, owing to the particular qualities demanded from the insulator, such as insulating capacity,

greater or smaller differences between the thermal expansion coefiicient of the binder and that of the base material are unavoidable, which leads to internal stresses in the insulator and reduces its stability at changing temperatures.

The present invention avoids this disadvantage by employing a binding agent which is flexible or yielding relative to the base material.

In cases in which the thermal expansion of the base material greatly increases with increasing temperature, as for example, in the case of corundum, it may happen that the thermal expansion of the base material gradually exceeds the thermal expansion of the binding agent. In such cases, it is preferable to employ a binding agent which, in the finished insulator, begins to become plastic when approaching the critical temperature range. By "becoming plastic, not such a softening of the binder is of course, meant as would cause a deformation of the insulator and render it useless. The plasticity of the binder need only be such as to prevent the rise of the internal stresses in the insulator beyond a certain point. This plasticity of the binder very considerably improves the stability of the insulator at changing temperatures when it occurs even at comparatively low temperatures, say, between 500 and 800 C. A binding agent having the above described qualities when used with corundum as the base material, is, for example, a mixture of 32.7 parts by weight of steatite or talcum, 43.3 parts by weight of kaolin, and 24.0 parts by weight of feldspar, the mixture of corundum and binding agent being prepared, moulded and burnt by known methods to produce the finished article.

I declare that what I claim is:

1. A spark plug insulator made from corundum and a ceramic binding medium which becomes plastic at a temperature between 500 and 800 C. comprising steatite, kaolin and feldspar.

2. A spark plug insulator made from corundum and a ceramic binding medium, comprising a mixture of .32.? parts by weight of steatite, 43.3 parts by weight of kaolin and 24.0 parts by weight of feldspar, which is flexible as compared with the corundum when the insulator is under stress due to heat expansion.

3. A spark plug insulator made from corundum and a ceramic binding medium which becomes plastic at a temperature between 500 and 800 C. comprising a mixture of 32.7 parts by weight of steatite, 43.3 parts by weight of kaolin, and 24.0 parts by weight of feldspar.

RICHARD STRIBECK. 

